In LTE (Long Term Evolution), radio base stations eNB may support different releases or functions. Thus, handover between radio base stations eNB requires control to avoid inconsistency in “configuration (setting)” between the radio base station eNB and a mobile station UE.
As an information element of “configuration” in “Handover Command” set by a handover target radio base station T-eNB, “delta configuration”, i.e., a difference from the current “configuration” is basically notified. This may result in a situation where the handover target radio base station T-eNB supporting an old release controls the mobile station UE without deleting the “configuration” of a handover source radio base station S-eNB supporting a later release. This situation is not desirable from the viewpoint of call control.
In order to avoid inconsistency in “configuration” between a mobile station UE and a handover source radio base station S-eNB, “full configuration” has been provided as a mechanism of once deleting all the “configuration” of the handover source radio base station S-eNB (see Non-Patent Documents 1 and 2).
When the mobile station UE receives “Handover Command” containing an information element of “full configuration”, the mobile station UE once deletes all the “configuration” (including releasing the bearer).
In addition, in the Inter-eNB handover, the handover source radio base station S-eNB forwards pending downlink data and uplink data to a handover target radio base station T-eNB. Thus, even when the mobile station UE moves, the communications can be continued.
Here, the Inter-eNB handover is specified such that an RLC-AM (Radio Link Control-Acknowledged Mode) function of the handover source radio base station S-eNB should perform the following operation in normal data forwarding (see Non-Patent Document 1).
In the downlink, the RLC-AM function of the handover source radio base station S-eNB is configured to forward or discard a UL-PDCP-SDU (Uplink-Packet Data Convergence-Service Data Unit) for which ACK has not been received in the RLC layer.
On the other hand, in the uplink, the RLC-AM function of the handover source radio base station S-eNB is configured to forward a UL-PDCP-PDU having been received in order of sequence numbers to a gateway node S-GW, and to forward, to the handover target radio base station T-eNB, or discard a UL-PDCP-PDU having been received out of order of the sequence numbers.